The present invention relates to improvements in vacuum forming or reduced-pressure moulding.
In vacuum forming or reduced-pressure moulding production methods known heretofore, a surface of a gas-permeable pattern is covered with an air-tight plastics film which has been previously softened by heating. The pattern is placed on a gas-permeable surface plate provided with an evacuating chamber, and evacuation is then effected so as to pull the plastics film tightly onto the surface of the pattern. For reduced-pressure moulding, a flask is placed over the surface plate to enclose the pattern and the plastics film fitted thereon. Solid particles, such as moulding sand, not containing a binder are then fed into the flask so as to surround the pattern, and these particles are aggregated together over the plastics film, which is then caused to adhere to the aggregated particles by evacuating the interior of the flask. The pattern is subsequently removed from the interior of the flask, whereby a mould cavity section covered with the plastics film is formed.
Also it is common known practice for efficient production from the moulds, to evacuate a plurality of mould flasks simultaneously by evacuating means normally comprising one or more vacuum pumps and a reservoir, and to move the mould flasks repeatedly through a mould shaping position, a pouring position, and a mould disintegrating position.
A mould produced according to the above-mentioned method requires a maxium air suction rate (degree of evacuation) when molten metal is poured into the mould. However, the mould does not require such a high degree of evacuation during short periods, for example a few minutes, respectively, before and after pouring of the molten metal. If a plurality of flasks are simultaneously evacuated by the evacuating means and these flasks are then moved sequentially through a mould shaping position, a pouring position and a mould disintegrating position and back to the initial mould shaping position as described above, then at any instant the air-tight film of the flask which has been moved to the mould pouring position is burnt out by contact with the molten metal, and thereby the air-tightness of this flask is reduced. Accordingly, the air suction rate from the interior of the flask to the evacuating means greatly increases, resulting in a lowering of the degree of evacuation in the other flasks which are also being evacuated by the evacuating means. Consequently, there has been a problem that moulds produced within the other flasks have been subjected to deformation or damage. To solve this problem, a method has been employed in which the evacuating capability has been enhanced by the use of a plurality of vacuum pumps as mentioned above, or by the use of a vacuum pump having a large capacity. However, even with this method, it has been found that the insufficient air suction rate cannot be supplemented, and especially in the case where the pouring speed is lowered, it has resulted in the production of defective cast products.